Documentation is not detailed enough for this item key parameter, it just says:

skip (skip processing of older data).

Without details zabbix users could understand that it works unconditionally, i.e. applied every time when agent gets list of active checks.
(it happened with me and several other people)

But actually, thankfully to lines:

	/* existing log[] and eventlog[] data can be skipped */
	metric->skip_old_data = (0 != metric->lastlogsize ? 0 : 1);

there is a condition - if this item previously has returned some values to server (thus "lastlogsize" is NOT 0 already), then the "skip" will be suppressed.
In other words the parameter works just for newly added items.

The logic has been implemented in very first commit r9669 of DEV-444 and it's still the same.

To be more user friendly I suggest to replace:

"skip processing of older data"

for doc versions 2.0-2.4 by:

"skip processing of older data, affects only newly added items which didn't return any value yet"

for doc versions 3.0 by:

"skip processing of older data, affects only newly added items"

at:
https://www.zabbix.com/documentation/3.0/manual/config/items/itemtypes/zabbix_agent (twice)
https://www.zabbix.com/documentation/3.0/manual/config/items/itemtypes/zabbix_agent/win_keys (once)

When updating pages for eventlog, I'd suggest also in Comments add an internal link "See also additional information on log monitoring." similarly as done for log, logrt keys comments.